This application claims benefit of Japanese Patent application No. 2000-340666 filed in Japan on Nov. 8, 2000, No. 2001-175204 filed in Japan on Jun. 11, 2001 and NO. 2001-271868 filed in Japan on Sep. 7, 2001, the contents of which are incorporated by this reference.
The present invention relates generally to a microscope zoom objective lens, and more particularly to a microscope zoom objective lens having a zoom ratio of 3 or greater.
A microscope object lens has a working distance, a numerical aperture (NA), etc. preset primarily in the range of about 1 to about 100 magnifications. Depending on how to observe, a variety of objective lenses inclusive of those capable of phase-contrast and fluorescent observations are kept on hand. For instance, objective lenses having a large numerical aperture at about 100 magnifications include an oil immersion objective lens and an objective lens provided with a correction ring. The correction ring objective lens system is designed to make correction for aberrations by moving a lens or lens group in the lens system depending on the thickness of a cover glass.
One typical example of the correction ring objective lens system is disclosed in JP-A 01-307717. The correction ring objective lens system is often used in the form of a system having a high magnification and a large numerical aperture. In such a high-magnification, large-numerical-aperture objective lens system, aberrations are likely to become worse due to thickness variations of a cover glass. For this reason, spherical aberrations are mainly corrected by moving a given lens group in the lens system along the optical system.
According to one approach to varying the viewing magnification of a microscope, an intermediate zoom optical system is located in the rear of an objective lens system for its attachment or detachment with respect to a viewing optical axis. With this approach, two different viewing magnifications may be obtained. Another approach makes use of an image-forming lens in combination with a zoom lens, as typically disclosed in JP-A 06-18784. This publication shows an arrangement wherein a zoom type image-forming lens is located in the rear of an objective lens.
U.S. Pat. Nos. 3,671,099, 3,456,998 and 3,421,897 disclose that a zoom lens is located in the rear of an objective lens.
Some stereomicroscopes, although lower in magnification than, and inferior in numerical aperture to, microscopes, are more improved thereover in terms of ease of manipulation and stereoscopy.
In a microscope objective lens system, a plurality of objective lenses are appropriately selected depending on magnification for observation. When the respective objective lenses have axial misalignments, low-magnification observation is changed over to high-magnification observation. In this case, however, a stage must be manipulated so as to adjust observation center misalignment, if any. When the objective lenses differ in parfocal length, it is required to manipulate an adjusting handle for working distance control. The need of providing for objective lenses corresponding to associated magnifications incurs some considerable cost.
In the correction ring objective lens system, the movement of lens groups is carried out mainly for the purpose of correction of spherical aberrations. Thus, the amount of movement of lens groups is slight, and so there is little or no change in viewing magnification.
A problem with the location of the intermediate zoom unit in the rear of the objective lens is that there is no improvement in resolving power because there is no change in numerical aperture even when viewing magnification is increased by zooming. A problem with the provision of a zoom unit between the objective lens and the image-forming lens is that the length of an optical system increases. For this reason, it is impossible to construct any compact microscope system. Another problem is that the position of an exit pupil varies largely with zooming.
The arrangement wherein a zoom type image-forming lens is located in the rear of an objective lens as set forth in JP-A 06-18784 is inferior in aberration performance to a conventional objective lens due to a limited number of fields. This arrangement is also less than satisfactory in terms of compactness and system performance due to the increased length of the zoom lens unit.
U.S. Pat. Nos. 3,671,099, 3,456,998 and 3,421,897 disclose that a zoom lens is located in the rear of an objective lens. However, these conventional arrangements are inferior to current objective lenses in terms of optical performance, e.g., aberration performance such as chromatic aberrations, spherical aberrations and flatness of field as well as specifications such as numerical aperture and the number of fields. Such optical performance cannot meet current market needs.
On the other hand, a stereomicroscope is lower in magnification and much smaller in numerical aperture than a microscope. At 10 magnifications as an example, the microscope has a numerical aperture of as high as 0.25 to 0.4 whereas the stereomicroscope has a numerical aperture of as low as about 0.1. Thus, when cells, etc. are observed, there is a noticeable difference in resolution. In short, the stereomicroscope is not comparable to a microscope objective lens in terms of resolution. In addition, afocal zooming for stereomicroscopes causes an exit pupil position to vary largely, and so place some considerable restrictions on systems when an illumination optical unit or a phototaking optical unit is incorporated therein.
A conventional variable-power method causes an exit pupil position to vary largely, resulting in performance deterioration such as shading and ambient light attenuation, shading in coaxial, vertical-incident illumination optical systems, etc. or restrictions on phototaking optical systems.
Thus, the optical systems so far proposed in the art have no similar numerical aperture and optical performance as in microscope objective lenses, and fail to provide any compact arrangements capable of increasing numerical aperture depending on magnification changeover with no need of selecting objective lenses.
In view of such problems with the prior art, an object of the present invention is to provide a compactly constructed microscope zoom objective lens system which has good-enough optical performance and a zoom ratio of at least 3. Another object of the present invention is to provide a microscope zoom objective lens system having a numerical aperture varying with a magnification change. Yet another object of the present invention is to provide a microscope zoom objective lens system with an exit pupil kept substantially constant (or with fluctuations of an exit pupil position being eliminated or reduced).
According to the first aspect of the present invention, there is provided a microscope zoom objective lens system comprising at least three lens groups; in order from an object, a first lens group having positive refractive power, a second lens group having negative refractive power and a third lens group having positive refractive power, wherein:
for zooming from a low magnification side to a high magnification side, said second lens group and said third lens group move along an optical axis of said microscope zoom objective lens system while a separation between said first lens group and said second lens group becomes wide and a separation between said second lens group and said third lens group becomes narrow, and
said first lens group includes at least one doublet consisting of a positive lens and a negative lens and having positive refractive power, said positive lens satisfying the following condition (1):
xcexd greater than 80xe2x80x83xe2x80x83(1) 
where xcexd is the Abbe consant of said positive lens.
According to the second aspect of the present invention, there is provided a microscope zoom objective lens system comprising at least three lens groups; in order from an object, a first lens group having positive refractive power, a second lens group having negative refractive power and a third lens group having positive refractive power, wherein:
for zooming from a low magnification side to a high magnification side, said second lens group and said third lens group move along an optical axis of said microscope zoom objective lens system while a separation between said first lens group and said second lens group becomes wide and a separation between said second lens group and said third lens group becomes narrow, and
said second lens group comprises at least two lens components which face each other at concave surfaces.
According to the third aspect of the present invention, there is provided a microscope zoom objective lens system comprising at least three lens groups; in order from an object, a first lens group having positive refractive power, a second lens group having negative refractive power and a third lens group having positive refractive power, wherein:
for zooming from a low magnification side to a high magnification side, said second lens group and said third lens group move along an optical axis of said microscope zoom objective lens system while a separation between said first lens group and said second lens group becomes wide and a separation between said second lens group and said third lens group becomes narrow, and
said third lens group comprises at least two lens components and includes at least one doublet made up of a positive lens and a negative lens.
According to the fourth aspect of the present invention, there is provided a microscope zoom objective lens system comprising at least three lens groups; in order from an object, a first lens group having positive refractive power, a second lens group having negative refractive power and a third lens group having positive refractive power, wherein:
for zooming from a low magnification side to a high magnification side, said second lens group and said third lens group move along an optical axis of said microscope zoom objective lens system while a separation between said first lens group and said second lens group becomes wide and a separation between said second lens group and said third lens group becomes narrow, and
said first lens group comprises a lens component located nearest to an object side of said microscope zoom objective lens system and being a doublet meniscus lens concave on said object side, said doublet meniscus lens consisting of, in order from said object side, a concave lens and a convex lens.
According to the fifth aspect of the present invention, there is provided a microscope zoom objective lens system comprising at least three lens groups; in order from an object, a first lens group having positive refractive power, a second lens group having negative refractive power and a third lens group having positive refractive power while the first lens group comprises a plurality of lens units, wherein:
for zooming from a low magnification side to a high magnification side, said second lens group and said third lens group move along an optical axis of said microscope zoom objective lens system while a separation between said first lens group and said second lens group becomes wide and a separation between said second lens group and said third lens group becomes narrow, and
the following conditions (2) and (3) are satisfied:
0.25xe2x89xa6D1/D0xe2x89xa60.7xe2x80x83xe2x80x83(2) 
0.05xe2x89xa6D2/D0xe2x89xa60.5xe2x80x83xe2x80x83(3) 
where D1 is the overall length of said first lens group, D2 is the amount of movement of said second lens group from said low magnification side to said high magnification side, and D0 is the overall length on the high magnification side of said microscope zoom objective lens system.
According to the sixth aspect of the present invention, there is provided a microscope zoom objective lens system comprising at least three lens groups; in order from an object, a first lens group having positive refractive power, a second lens group having negative refractive power and a third lens group having positive refractive power, wherein:
at least one aspheric surface is included and the following condition (19) is satisfied:
NAxe2x89xa70.5xe2x80x83xe2x80x83(19) 
where NA is the numerical aperture of said microscope zoom objective lens system at a high magnification side thereof.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent form the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.